Micro-mirror array devices are devices comprise a plurality of microscopically small mirrors arranged in an array. Such devices comprise micro-electromechanical systems (MEMS) devices whose states are controlled by a voltage between electrodes located around the array.
Micro-mirror array devices are operated to tilt along a certain axis (or axes) in order to deflect incident light. Typically, the tilt of the micro-mirror is controlled by the actuation of electrodes associated with the micro-mirror, for example, by using an applied voltage.
Characterisation of voltage against tilt angle for a given micro-mirror device is important in evaluating its performance. Furthermore, this relationship of voltage against tilt angle is also important in calibrating a micro-mirror for use in a certain application, for example, in “smart” lenses where micro-mirrors are used with variable focal length lenses and/or zoom lenses. In addition, obtaining or characterising the voltage-tilt angle relationship at run-time is often desirable to support run-time calibration.
US-A-2008/0137173 discloses a discretely controlled micro-mirror array device including a plurality of micro-mirrors in the form a micro-mirror array and a substrate including control circuitry. Each micro-mirror comprises a structure having a reflective surface with a plurality of segmented electrodes arranged on the substrate, the segmented electrodes being arranged to be evenly distributed or unevenly distributed with respect to their associated micro-mirror. Each micro-mirror is capable of both rotational and translational movement with multiple degrees of freedom. The activation of the segmented electrodes attracts different portions of the micro-mirror structure to provide a desired surface profile.
However, by segmenting the electrodes and therefore increasing the number of electrodes to control each micro-mirror, more complex electronic circuits are required to actuate and control each of the micro-mirror elements to provide the desired surface profile.
WO-A-2009/032347 describes a micro-mirror array device comprising a plurality of micro-mirror array elements. Electrodes associated with the micro-mirror elements are shaped to act as stoppers to limit the movement of the micro-mirror elements when actuated by an applied voltage.
By using the electrodes as stoppers, charge build-up becomes a problem during operation of the micro-mirror.
In current micro-mirror array devices, all the micro-mirror elements in the array are typically identical, with the one micro-mirror element being optimised and then replicated throughout the entire array. Micro-mirror array devices designed this way have the disadvantage that the accuracy of tilt angle required for some implementations is quite high and a complicated and precise manufacturing process is needed to achieve the desired high resolution.
In addition, although each micro-mirror element is designed to be symmetrical about its pivot point or tilt axis, in many applications, the micro-mirror element has asymmetrical performance about its pivot point or tilt axis. This asymmetry cannot be adjusted when each micro-mirror element is designed to be the same.
Moreover, in many implementations of micro-mirror arrays, the electrodes are used for actuation and measurement, and as described above, in some cases, as stoppers to limit the range of movement of each micro-mirror element.